The present invention relates to superconductor cables, and more particularly, to superconducting cables having controlled interstrand resistance.
The most common application for superconducting cables is in the manufacture of electromagnets, but this use encounters a significant problem as a result of eddy currents generated within the cable and their deleterious effects produced on magnetic fields and on critical currents. As field strength increases, lateral voltages are generated in the cable which produce eddy current loops extending laterally through normally conducting layers from one superconducting strand to the next. It is these eddy currents which impair performance of the superconductor.
Superconductors fabricated of strands of niobium/titanium filaments in a copper matrix and overall copper jacket are known in the art. When these conductors are used in compacted cables for AC magnet applications, it has been found that the heat and pressure used in fabrication frequently causes the strands to have low electrical resistance to one another, thereby adversely affecting the magnet's AC loss characteristics.
One of the first attempts to solve this problem was to replace the outer copper shell of the wire strands with a more resistive material such as a copper/nickel alloy. Although some strand decoupling was achieved, the disadvantages were numerous and included: difficulty and expense in fabrication, restrictions in the copper-to-superconductor ratio, and a limited and fixed value for the amount of decoupling which could be achieved.
Coating the superconductor strands with a semiconductive material is currently the most accepted method of obtaining the desired interstrand resistance. One material that has been extensively used to date is a tin/silver alloy applied by a hot dip process involving passage of wire through a bath of the molten alloy; one type is sold by Tory Corporation under the trademark Staybrite.RTM.. However, this material has some limitations. For example, the tin/silver coatings do not provide the required amount of interstrand resistance in many instances. A typical value of R.sub.I between strands obtained is 2 microhms. Additionally, if the coating process is not performed carefully, the superconducting properties of the wire may be adversely affected due to overheating. Further, the coating thickness is not well controlled in this process and can vary from 10 to over 100 microinches over the course of a run. The low softening temperature of tin/silver is also a problem when curing some of the higher temperature insulation systems used on these cables. The soft surface coating can flake off and interfere with the extremely precise dimensional measurement of the cable made just after it is formed.
The most favored material used to date to provide the required interstrand resistance is black copper oxide; one type is sold by Enthone Corporation under the trademark Ebonol C.RTM.. This material is applied by running copper-clad superconducting wire through an oxidizing agent to form a coating of cupric oxide on the surface. Recent tests, however, have shown that values of R.sub.I between strands can vary from 10,000 microhms to as much as 2 ohms and also appear to be nonlinear with applied current. Further, a copper oxide coating is an extremely difficult and expensive coating to provide, and the chemicals involved are corrosive and must be treated as a hazardous waste. The coating thickness is not well controlled which probably contributes to the nonlinearity in strand to strand resistance R.sub.I. A further concern is that the coating itself is abrasive and quite powdery which causes a number of difficult problems in manufacturing and measuring the compacted cables.
It is an object of the present invention to provide a novel method for making a superconductor cable with high interstrand resistance to control eddy current losses.
It is also an object to provide such a superconductor cable which may be processed at high temperatures and pressures.
Another object is to provide such a superconductor cable which will have substantially consistent and stable interstrand resistance generally linear with current flow through the cable.
A further object is to provide such a superconductor cable which may be fabricated readily and economically using known processes.